A group of researchers in Brazil recently looked at the impact that age of mares and stallions when bred might have on the number of male vs. female offspring. Two different trials were done, one from a randomly obtained population of horses (59,950 horses born from 1990-2011), the other based on data from 253 horses of several different breeds born into a herd from 1989 to 2010. The first trial, with the large number of horses, showed the pattern of distribution of the sex of the offspring according to the age of the parent. The second trial, with the lower number of horses, confirmed the influence of the age of the mare on the sex of the offspring. In summary, as the mare’s age increased, so did the probability that she would produce a filly.
Stallions, ages 10-20, also showed the same trend, but once they reached 20 and older, the ratio of colts to fillies returned to 50:50. Older stallions mated to younger mares produced more males, while older mares mated to younger stallions were more likely to produce females (esp. of the stallions were 10-15 years younger). The researchers concluded that “the parental age affected the offspring sex ratio in horses and that this effect was stronger for the mares than for the stallions.”
Relevance: Fetal sexing, done by ultrasound during specific times of gestation, has provided a means by which we can get a sneak preview of what our broodmare will produce at the end of her gestation. Utilizing data such as this, where we can potentially impact the sex of a foal by considering the age of the mare and stallion, may be a way to “stack the cards in our favor” to increase the odds of producing that colt or filly that we have been wanting for so long.
Scientific Article: Santos MM, et. al. “Sex ratio of equine offspring is affected by the ages of the mare and stallion”. Theriogenology, 2015 Oct. 15;84(7): pp. 1238-45.
Companion Article: Bazay, Casie “Does Dam or Sire Age Affect Offspring Gender?” the horse.com, Oct. 17, 2015, article #36577.